Drug addiction is a brain disorder characterized by a progression toward compulsive drug use and relapse to drug seeking during abstinence. The primary goal of this new application is to enhance our understanding of the neurobiological and neurochemical mechanisms involved in methamphetamine abuse. The nucleus accumbens (NAc) is a brain region in a complex circuit that mediates initial drug reward and relapse during periods of abstinence. In the NAc, subtypes of dopamine (DA) and adenosine (ADO) receptors are co-localized in distinct subpopulations of neurons where they play antagonistic roles on cellular functioning. Thus, neurons having co-localization of either D1/A1 or D2/A2A receptor subtypes are known to form distinct output pathways, which influence specific aspects of behavior. The opposing actions of DA and ADO receptor subtypes can be mediated by direct physical interactions (i.e. heteromeric receptors), and/or through differential activation of G- protein mediated signaling cascades. How these opposing receptor subtypes localized to distinct neuronal populations regulate addictive behavior is unknown. We have evidence to suggest that stimulation of ADO A1, but not A2A, receptor subtype inhibits methamphetamine reinforcement and relapse. These effects differ from our findings that cocaine relapse is inhibited by non-selective stimulation of A1 and A2A receptors. Our overarching hypothesis is that chronic methamphetamine use specifically disrupts ADO A1 receptor signaling in the mesolimbic DA pathway, leaving DA D1 receptors unregulated contributing to methamphetamine reinforcement and relapse. In Aim 1, experiments will assess methamphetamine-induced changes on ADO receptor subtypes within the mesolimbic system. Additional studies will identify how methamphetamine intake alters the heteromeric interactions between ADO and DA receptor subtypes. Experiments in Aim 2 are designed to dissect the differential influence of specific ADO receptor subtypes on methamphetamine reward and reinforcement using place conditioning and progressive ratio responding, respectively. Aim 3 is designed to explore how ADO receptor subtypes may differentially influence reinstatement to methamphetamine seeking. Additional studies will explore how the differential influence of ADO receptor subtypes interact with methamphetamine seeking induced by specific DA receptor subtypes in the NAc. Together these studies offer the potential to better our understanding of the brain mechanisms involved in methamphetamine abuse that appear to be substantially different than mechanisms associated with another abused psychostimulant, cocaine. These studies offer the potential to create novel treatment strategies such as A1 receptor agonists or bivalent receptor ligands (e.g. D1 antagonist-A1 agonist) that could specifically target heteromeric receptors localized to specific subpopulations of neurons within specific neural circuits that undergo methamphetamine- induced alterations.